In a previous study9,10 HeLa cultures were inoculated with herpes simplex virus and nourished with growth medium which contained neutralizing antibody. These cultures could be serially subcultured without loss of the cells or elimination of the virus as long as the growth medium contained a high level of neutralizing antibody. At the 17th subculture of one group of cultures it was noted that the appearance of the plaques of infected cells had changed. The plaque was no longer made up of small multinucleated giant cells and rounded cells lying singly or in clumps but was composed almost entirely of a plaque of large multinucleated syncytial giant cells. Since preliminary cross neutralization tests indicated the virus which produced the syncytial giant-cell plaque was herpes simplex virus, it was decided to study this strain in more detail.

The history of the origin of the syncytial giant-cell-forming strain of herpes virus follows